This invention relates to modulated infrared sources and more particularly and generically to modulators in which spatial modulation is provided.
The modulation of infrared radiation, due to its long wavelength, has not been accomplished easily in the past. Problems in modulation of infrared energy include those of unwanted refraction due to the long wavelengths as well as absorption of the energy by the apparatus utilized in the modulation technique. Absorbed energy is reradiated in a diffused pattern thereby in many instances degrading the modulation. Additional problems center around materials which can withstand the infrared radiation while the same time being sufficiently light weight and structurally stable enough to withstand cyclic motion normally employed in the production of a modulated beam. It will be appreciated that when IR sources include heated elements, modulation of the energy to the element is ineffective to cause modulation of the radiation from the element due to the long heating and cooling cycles inherent with the IR sources in which elements are heated.
One of the most important applications of modulated infrared sources is in the area of infrared countermeasures. In this application, the modulated infrared source is employed to render ineffective heat seeking missiles which home in on the heat generated by the engines which propel the target at which the missile is aimed. These engines include internal combustion engines, jet engines, rocket engines or the like.
In general, it is the purpose of the infrared countermeasure device to produce a modulated infrared signal of sufficiently high intensity to blanket or mask the infrared output from the above mentioned engines. Modulated infrared sources exist in the prior art which employ IR sources with temporal modulators for this purpose. In one embodiment temporal modulation involve the so-called xe2x80x9cchopperxe2x80x9d technique, in which apertures spaced from the source are sequentially covered and uncovered in a shutter technique. However, in these sources when the apertures are covered energy radiated from the IR source is either absorbed by the occluding member or reflected back into the source at a non-optimum angle such that this energy is lost. Where energy for the IR source is virtually unlimited such as is the case when fuel is burned for the production of infrared radiation, temporal modulation techniques work well. Temporal modulation of electrically powered sources also works well where sufficiently large amounts of electrical power are available as in jet powered fighter aircraft. However, when the IR source must depend for its energy on electrical power which is critically limited, it is desireable that as much of the energy from the IR source as possible be utilized in order that the infrared source radiate sufficient energy to blanket or mask the infrared energy from the taget""s engine.
Moreover, to provide omnidirectional or near omnidirectional coverage the infrared source must be omnidirectional so as to be able to counter measure heat seeking missiles coming in from any direction. In the prior art omnidirectional coverage has been obtained by the provision of a large number of apertures about the IR source. The modulation is obtained by the rotation of a cylindrically shaped mask in front of the apertures. While these systems are effective where unlimited power is available, the provision of temporal modulation presents a problem of efficiency which can be critical in many applications because the radiation from the source may be blanketed or masked by radiation from the target.
Moreover, due to the limited power available the coupling of 100% of the power from the IR source out of the source is so critical that the shape of the projected infrared image becomes exceedingly important. Assuming a line source, which is effectively an omnidirectional radiator, it has been found that refractive optics which would ordinarily focus and couple out a great deal of the energy from the IR source suffer from the fact that the radiated image is not linear. If the image is considered to be rectangular, with the use of refractive optics, the long sides of the rectangle are bent inwardly in a concave manner. Thus, maximum intensity appears at the center of the rectangular image, with the energy being somewhat reduced towards the ends of the image. When energy levels are critical the refractive optics may result in a situation where the energy from the infrared source is enough less than that of the energy from the target""s engines such that the infrared source is blanketed or masked by the infrared energy from the target rather than the other way around. The ratio of infrared energy from the source vis-a-vis infrared energy from the target is called the jam-to-signal ratio and this ratio is a measure of the effectiveness of the infrared source as a counter-measure. It will be appreciated that if this ratio is greater than 1, the infrared source can be effective as a countermeasure.
The above problems are solved by the subject invention in which close to 100% of the infrared energy is coupled out into space by xe2x80x9cspatial modulationxe2x80x9d. The term xe2x80x9cspatial modulationxe2x80x9d as used herein refers to the sweeping of an infrared beam past a point in space removed from the infrared source a number of times per second, corresponding to the frequency of the modulation. In one embodiment of the subject invention this is accomplished by rotating the focusing optics about a stationary infrared source at an rpm commensurate with the modulation frequency desired. The optics in the preferred embodiment are reflective optics in which a narrow parabolically shaped reflector surrounds an infrared source such that a beam is formed in which a portion of the energy in the beam comes directly from the IR source, with the other portion being reflected. Direct radiation in the beam is important because it contributes substantially to the coupling of close to 100% of the radiation from the source to a distant point. The use of a narrow parabola also permit the generation of a highly defined beam with unusually sharp edges. The source is located at the focus of the reflector and the reflector is rotated about its focus. Multiple reflectors may be provided to provide multiple beams.
It is therefore an object of this invention to provide an improved modulated infrared source in which reflective optics are rotated about an IR source to produce a beam which sweeps by a point in space with the beam formed having a portion of its energy coming directly from the IR source.
It is a further object of this invention to provide modulation of an infrared source by the revolving of parabolically contoured focusing optics about the infrared source.
It is another object of this invention to provide a spatially modulated infrared source in which a number of infrared sources are each located in a different reflective cavity of a housing which is rotated with the reflective cavities forming the focusing optics.
It is a further object of this invention to provide reflective focusing for an infrared source in which the beam formed by the focusing means is extremely sharp and well defined in the infrared region of the electro-magnetic spectrum and in which some energy in the beam comes directly from the infrared source.
It is a yet still further object of this invention to provide a modulated omnidirectional infrared radiation source isotropic in azimuth.